A conventional preamplifier for radiation detection is shown in FIG. 5. The waveform of the output voltage from the preamplifier is shown in FIG. 6. Another conventional radiation detection preamplifier which superimposes a reset pulse on the DC bias voltage applied to a radiation detector is shown in FIG. 7. A further conventional radiation detection preamplifier which applies a reset pulse to the base of a PNP transistor is shown in FIG. 8.
In a radiation detector using a semiconductor such as silicon or germanium, if photons enter the depletion layer, ionization occurs. As a result, electron-hole pairs are generated according to the energy of the incident photons. A given bias is applied to the radiation detector. A feedback circuit which has a preamplifier together with an electrostatic capacitor integrates the output signal from the detector. In this way, a voltage is taken which varies in a stepwise fashion according to the amount of radiation. A charge integration-type preamplifier as shown in FIG. 5 is used as the above-described preamplifier. An N-channel FET 22 is used at the first stage of a radiation detector 21. A non-inverting amplifier 23 having a large degree of amplification is connected with the drain terminal D of the FET 22. The output signal from this amplifier 23 is fed back to the gate G of the FET 22 via an electrostatic capacitor C.sub.f having a capacitance of C.sub.f.
The polarity of the bias voltage applied to the radiation detector 21 is made positive or negative, depending on the material and on the structure of the detector. The present invention pertains to a preamplifier used in a detector employing a positive bias voltage.
Let E be the energy of photons impinging on the radiation detector 21. The number of electron-hole pairs produced inside the detector 21 is E/W, where W is the energy needed to generate one electron-hole pair. The produced electric charge q (E/W) is driven off by the bias voltage and stored in the electrostatic capacitor C.sub.f, it being noted that q is the charge of an electron. The stored charge reduces the output voltage V.sub.o from the non-inverting amplifier 23 by q (E/W)/C.sub.f. Therefore, if photons successively enter the radiation detector 21, the output voltage V.sub.o assumes a staircase waveform having a height corresponding to the energy of the photons, as shown in FIG. 6. A signal-processing unit 24 once differentiates this staircase waveform and then integrates it to produce pulses having heights corresponding to the energies. The radiation is counted by a pulse-height analyzer. When no photon enters, the output voltage V.sub.o gradually drops, because the electric charge due to leakage current from the radiation detector is stored in the electrostatic capacitor C.sub.f. For this reason, if the output voltage V.sub.o decreases greatly, a saturation takes place. Under this condition, it is impossible to detect radiation. Accordingly, in order to continuously detect and measure radiation, it is necessary to set a threshold voltage V.sub.r before the cutoff voltage V.sub.c is reached. The stored charge is released at this threshold voltage to permit continuous operation.
One conventional preamplifier which continuously releases the electric charge stored in the electrostatic capacitor C.sub.f is shown in FIG. 5(a), where a resistor R.sub.f is connected in parallel with the electrostatic capacitor C.sub.f. The addition of the resistor R.sub.f increases resistor noise. Therefore, this network is not suited for a preamplifier used in low-noise applications. Accordingly, a preamplifier as shown in FIG. 5(b) is needed. In particular, when a certain amount of electric charge is stored in the electrostatic capacitor C.sub.f, a switch S is closed to electrically discharge the capacitor. This is known as the "reset" function.
This preamplifier has been proposed by V. Radeka, "Charge Amplification Without Charge Leak Resistor," IEEE Trans., Nucl. Sci., NS-17, No. 3, pp. 433-439 (1970). In this preamplifier, as shown in FIG. 7, a given recurrent positive going pulse is superimposed on the DC bias voltage applied to the radiation detector 21 for resetting purposes. The positive pulse voltage is applied to the gate G of an FET 22 via an electrostatic capacitor C.sub.d connected with the radiation detector 21. This increases the output voltage from this FET. If the amplitude of the pulse voltage is made sufficiently large, then the gate G is placed in a positive potential. As a result, the junction between the gate G and the source S of the FET 22 is biased forward and the FET conducts. Hence, the electric charge in the capacitor C.sub.d is released to the source S. Then, if the reset pulse is restored, the charge in the capacitor C.sub.f flows into the capacitor C.sub.d. In consequence, the charge stored in the capacitor C.sub.f is also released. In this case, to reduce disturbance in the circuit caused by the addition of pulses of a single polarity, pulses of positive polarity and pulses of negative polarity may both be used.
Another preamplifier used for the same purpose has been proposed by F. S. Goulding et al., "An Improved Operating Mode for A Si(Li) X-Ray Spectrometer," IEEE Trans., Nucl. Sci., NS-37, No. 2, pp. 171-176 (1990). This preamplifier is shown in FIG. 8. On resetting, a negative pulse is applied to the base of a PNP transistor 25 forming a cascode circuit so that the junction between the gate G and the drain D of an FET 22 is forward-biased. In this way, the drain voltage of the FET 22 is urged to drop. The circuit applying the pulse is so designed that the output voltage V.sub.o drops as electric charge is stored in the electrostatic capacitor C.sub.f and that the circuit produces a trigger pulse for resetting the capacitor when a predetermined threshold voltage is reached. In this manner, the base drive circuit is operated.
The former method of superimposing positive going pulses on the DC bias voltage and adding the resulting voltage to the detector 21 has the following two disadvantages. One is that a pulse voltage having a large amplitude is needed because the electrostatic capacitance C.sub.d of the detector 21 is small. Another disadvantage is that noise may enter via the circuit which adds the pulse voltage to the detector 21. These disadvantages are great impediments in putting the preamplifier into practical use.
The latter method requires that a large driving pulse be applied to the base of the PNP transistor as pointed out by the proposer himself. Therefore, the reset response is slow. Hence, this preamplifier is unsuited for measurements at high counting rates.